The present invention relates to an analog-to-digital (A/D) converter for converting signals from a plurality input channels into digital signals, and in particular, to an A/D converter included in a microcontroller.
Due to development of the large scale integration (LSI) technology, microcontrollers today include various peripheral circuits integrated therein. Particularly, in microcontrollers adopted in a car control system, an A/D converter to conduct an analog-to-digital conversion for analog signals attained from a large number of sensors has become important.
In an A/D converter of the prior art, techniques have been devised to convert analog signals from a plurality of analog input channels into digital signals without imposing load onto a central processing unit (CPU).
For example, JP-A-6-149479 and JP-A-2002-314418 describe conventional techniques in which to set a selection pattern of analog input channels for an A/D conversion, an A/D conversion table is disposed to store therein a plurality of analog input channels. When an A/D conversion is initiated by, for example, a timer, each analog input channel is selected in an order set in the A/D conversion table to conduct the conversion.
JP-A-8-162953 describes a conventional technique of an A/D converter including a sequential A/D conversion mode to repeatedly conduct an A/D conversion for one or more analog input channels. To store a result of the conversion in a conversion result register, the A/D converter includes a conversion result transfer circuit.
The A/D converter is effective in that an A/D conversion is conducted for signals from a plurality of analog input channels without imposing load onto the CPU and the conversion results are stored in a predetermined location.
On the other hand, many systems such as a car control system increasingly require various functions and hence the CPU has a higher processing speed to execute many tasks. This leads to a tendency to increase the selection patterns of analog input channels in the A/D conversion. However, the prior art requires an A/D conversion table of large capacity to store therein a large number of analog input channel numbers each of which includes a plurality of bits. Therefore, a storage of large capacity is additionally required to integrally store a large number of A/D conversion tables. For example, when the number of analog input channels is 2n and an analog input channel number includes n bits, an A/D conversion table to set therein the analog input channel numbers of 2n channels requires 2n×n bits.
An A/D converter integrally installed in a microcontroller is applied to a plurality of microcontrollers for respectively different uses in many cases. There exists also a microcontroller in which a low cost thereof takes precedence over its functional variety. Therefore, an A/D converter having only fundamental functions is employed in this case. However, it is required in the prior art to dispose constituent elements such as A/D conversion tables and a conversion result transfer controller in the A/D converter. It is therefore difficult to share the A/D converter among various microcontrollers.